(1) Field of the Invention
The present invention relates to a propylene-ethylene/butene block copolymer having an excellent softness and a high transparency and having no tackiness.
(2) Description of the Related Art
Conventional molded articles formed of an ethylene-propylene rubber (hereinafter referred to as "EPR") are defective in that they have a surface tackiness. EPR prepared by polymerization of ethylene and propylene is an agglomerate body and therefore, the agglomerate body is difficult to handle. As means for overcoming these disadvantages, there has been proposed a method of increasing the molecular weight of EPR.
However, a molded article composed of EPR having a high molecular weight is defective in that the heat resistance, processability and mechanical strength are not sufficient. It is known that this problem can be solved by blending polypropylene into EPR. However, a molded article composed of a mixture of EPR and polypropylene is white or milky white and is inferior in transparency to other resins having an equivalent softness, such as linear low-density polyethylene and soft polyvinyl chloride. Therefore, a mixture of EPR and polypropylene cannot be used for vessels for which transparency is required, such as a sheet, a film and other molded articles.